1. Field of the Invention
The present invention relates to a marine drive, and in particular to a propulsion system for a marine drive.
2. Description of the Related Art
Many watercraft employ outboard motors that are mounted on the aft end of the watercraft. An outboard motor generally includes a power head that houses an engine, a drive shaft housing situated below the power head, and a lower unit that is positioned below the drive shaft housing. The lower unit typically houses a transmission and a propulsion shaft that drives a propulsion device.
Some outboard motors employ counter-rotating propeller systems which utilize a pair of counter-rotating propellers that operate in series about a common rotational axis. By using propeller blades having a pitch of opposite hand, the dual propeller arrangement provides significant improvement in propulsion efficiency.
Prior propellers used with counter-rotating propeller systems tend to suffer from a number of drawbacks, however. Prior designs tend to be overly complicated, being formed by a multitude of components. For instance, a propeller often includes a propeller hub positioned about an inner sleeve that attaches to a propeller shaft. A rubber coupling couples the propeller hub to the inner shaft. At least one spacer is usually used to support the hub about the propeller shaft beyond a rear end of the inner sleeve. The use of all of these individual components complicates the assembly of the propeller and adds to labor and material costs.
Some conventional propeller design also use the rubber coupling to support at least a portion of the propeller hub about the inner sleeve. In this use, at least a portion of the rubber coupling undergoes cyclic tension and compression loads. The rubber eventually fatigues under these stresses and collapses into itself. The collapsed rubber coupling consequently looses a certain degree of its elasticity and no longer effectively absorbs rotational vibrations within the propeller.